So far, various positioning systems have been proposed. FIG. 1 shows a comparison of various kinds of positioning systems with the ordinate as positioning accuracy and the abscissa as the spacing between equipment required for positioning.
Positioning systems typified by GPS (Global Positioning System) perform time synchronization by using an accurate clock such as an atomic clock and measure the difference in arrival times of radio waves to realize positioning. Problems with a positioning system utilizing an atomic clock, such as GPS, include: high equipment cost due to the usage of an atomic clock: difficulty in underground usage because of the necessity of synchronization with the atomic clock on the satellite; large power consumption in high accuracy positioning and positioning without the aid of base stations; and others.
In positioning systems based on RFID (Radio-Frequency Identification) tags and PHS (Personal Handy-phone System), radio wave intensity is used for positioning. In such a technique, the position of a subject is determined assuming that the subject is present near one of the installed RFIDs and base stations from which the subject can receive the strongest radio wave. Moreover, an approximate distance is determined from the intensity of arriving radio wave. A problem of this technique is that since positioning accuracy becomes approximately the same as that of the base station installation spacing between the positioning equipment, many pieces of positioning equipment need to be provided thereby leading to high deployment cost.
In systems utilizing a wide bandwidth such as wireless LAN (Local Area Network) and UWB (Ultra-Wide Band), positioning is performed by comparing the difference in arrival time between the radio waves from a target radio station to be measured and a reference radio station. In this respect, wideband communication with sharp time changes is suitable for timing measurement. A problem with a positioning system utilizing a wide bandwidth is that a larger bandwidth generally results in larger reception power, and that since the accuracy is determined by the inverse of the bandwidth, use of wireless LAN does not provide enough bandwidth thereby resulting in poor accuracy.
Although impulse-UWB, whereby a high accuracy can be expected, can suppress transmission power to a very low level, it requires larger reception power and therefore is not suitable for systems which are driven by a battery for long hours. In order to cover such deficiency, a technique is envisioned in which a UWB for the transmission to a base station is combined with another technique for the reception from a base station. However, in a UWB which utilizes a wide bandwidth, power output needs to be suppressed to a low level to avoid interference with other radio waves, and therefore an arrival distance of about 10 m is assumed in UWB standards typified by IEEE 802.15.4a. Therefore, when only the communication from the target radio station for measurement to a base station is performed by UWB, the range within which positioning is possible becomes necessarily about 10 meters.
Besides the aforementioned positioning systems, there is a technique in which the difference in arrival time between a sound wave and a radio wave is used to perform distance measurement. Problems with this technique are that only sound waves may be interrupted, and that a microphone and a speaker are separately needed.
Other than those techniques shown in FIG. 1, there are a laser interferometer which utilizes a reflected wave to perform distance measurement in units of μm, and a radar which utilizes a reflected wave and a wide bandwidth to perform distance measurement. The problems with the use of reflected waves are that large power for transmission and high sensitivity for reception are required, and that since a circulator or the like is required to separate transmission and reception, the size of housing will become large.
Among those, a system which performs distance measurement without using an atomic clock, a base station synchronized with an atomic clock, a reflected wave, a sound wave, and a wideband communication is proposed in Patent Document 1 (Japanese Patent Laid-Open No. 11-178038).
FIG. 2 is a block diagram to show the configuration of the positioning-capable mobile communication system according to Patent Document 1.
An audible sound signal or data signal, which is a positioning signal, is transmitted from positioning signal originating section 811 of positioning apparatus 801 to the speech channel of mobile radio terminal apparatus 804. Next, the positioning signal transmitted from positioning apparatus 801 is turned back by turn-back means of speech section 843 of mobile radio terminal 804 and is returned to phase detection section 812 of aforementioned positioning apparatus 801. Eventually, phase detection section 812 compares the phases of the received turn-back signal and the source positioning signal of positioning signal originating section 811 to measure a delayed phase, and notifies the measurement result to calculation process section 814. Calculation process section 814 calculates a space propagation distance between radio base station 803 and mobile radio terminal apparatus 804 from the delayed phase.
Patent Document 1 neither specifically describes the turn-back means, nor the method of calculating distance from phase. Although the description is made assuming a PHS system, the space wavelength of the PHS frequency (1900 MHz band) is about 16 cm meaning that the same phase will be acquired for about every 8 cm even if turning back is performed by a certain technique, and therefore positioning cannot be practiced without specific description of the method of distance calculation. Moreover, since description is made based on the assumption of a PHS system which is essentially based on TDMA-TDD (Time Division Multiple Access/Time Division Duplex), it is not even clear whether transmission and reception are performed concurrently or not.
Now, suppose transmission and reception are performed concurrently, and turning back is realized by a certain technique, it is inferred after the analogy of a CW radar system that transmission/reception separation means based on a circulator is utilized in transmission/reception sections 831 and 841. Since the circulator used herein is of a large size, it is not suitable for small terminals.
Further, in the distance calculation by the mobile communication system of Patent Document 1, since each mobile radio terminal apparatus will use a communication control center, it is not possible for each mobile radio terminal apparatus to directly measure the distance to another mobile radio terminal apparatus.
FIG. 3 shows the positioning technique by the positioning-capable mobile communication system according to Patent Document 2 (Japanese Patent Laid-Open No. 2006-42201).
In Patent Document 2, the configuration is such that two carrier waves are transmitted from the transmission side and the phase difference between them is measured at the reception side to perform distance measurement. It is noted here that since the phase difference is generated from the frequency difference, a long wavelength (difference) can be employed unlike Patent Document 1.
Hereafter, explanation will be made according to the description in paragraphs 52 to 72 of Patent Document 2.
In FIG. 3, the ordinate shows the amplitudes of the first and second carrier waves and the abscissa shows distance. Symbol R represents the distance from a mobile terminal of transmission side to a mobile terminal of reception side. At the mobile terminal of transmission side, the first and second carrier waves are synchronized. Therefore, the phases of the first and second carrier waves are in agreement with each other at the mobile terminal of transmission side. Δφ indicates the phase difference between the first and second carrier waves at the mobile terminal of reception side, where −π≦Δφ≦π.
Hereafter, a method of calculating distance R from a mobile terminal of transmission side to a mobile terminal of reception side, based on phase difference Δφ between the first and second carrier waves will be described.
Suppose the velocity of radio wave is c, the wavelength of the carrier wave is λ, the frequency of the carrier wave is f, and the period of carrier wave is T, the following equation holds:c=λ/T=λf  (1)
From above equation (1), the angular frequency w of the carrier wave is given as follows.ω=2π/T=2πf  (2)
Distance R is represented by phase as 2πR/λ [rad].
From above equation (1), phase is represented as the following equation:2πR/λ=2πRf/c  (3)Here, the first and second carrier waves at the mobile terminal of transmission side are represented by equations (4) and (5):w1T=sin(2πf1t+φ1)  (4)w2T=sin(2πf2t+φ2)  (5)
In above equations (4) and (5), w1T and w2T are respectively the amplitudes of the first and second carrier waves at the mobile terminal of transmission side, t is time, and φ1 and φ2 are respectively the phases of the first and second carrier waves at the mobile terminal of transmission side.
From above equations (3), (4) and (5), the first and second carrier waves at the mobile terminal of reception side can be represented respectively by equations (6) and (7):w1R=sin(2πf1t−2πRf1/c+φ1)  (6)w2R=sin(2πf2t−2πRf2/c+φ2)  (7)
In above equations (6) and (7), w1R and w2R are respectively the amplitudes of the first and second carrier waves and t is time, at the mobile terminal of reception side.
At transmitter 1, the first and second carrier waves are synchronized and therefore φ1=φ2.
Therefore, from above equations (6) and (7), phase difference Δφ between the first and second carrier waves at the mobile terminal of reception side are given as follows:Δφ=2πR/c(f1−f2)=2πR/c·Δf  (8)
In above equation (8), Δf is the difference between first frequency f1 and second frequency f2. The above equation (8) may be modified into the following equation:R=(c/2π)·(Δφ/Δf)=(cΔφ)/(2πΔf)(−π≦Δφ≦π)  (9)
Here, suppose a case in which difference M between first frequency f1 and second frequency f2 is set to be 1.0 MHz. In this case, when phase difference Δφ becomes π, distance R is calculated from above equation (9) as follows:R=(3.0×108×π)/(2π×1.0×106)=150[m]
What is described so far is the explanation described in Patent Document 2. Here, a problem with the technique of Patent Document 2 lies in equation (8). Since phase difference Δφ of the carrier waves at the mobile terminal of reception side, which is obtained by subtracting the coefficient of the sine term in equation (7) from the coefficient of the sine term in equation (6), must beΔφ=2π(R/c−t)·(f1−f2)  (10)and therefore Δφ will change in time, distance cannot be calculated without time information (time at the moment when φ1=φ2 is satisfied at the transmission side) and therefore, in fact, the technique disclosed in Patent Document 2 cannot be practiced without time information.
Moreover, when a case is assumed in which the aforementioned technique can be practiced, two different frequencies are supposed to be transmitted concurrently. When two different frequencies are transmitted concurrently, the peak power will be twice as large as the average power. Since the transmission/reception system is designed in accordance with the peak power, when the difference between the average power and the peak power increases, power consumption for the same transmission power will increase. This is also true with wideband modulation schemes such as CDMA and OFDM.    Patent Document 1: Japanese Patent Laid-Open No. 11-178038    Patent Document 2: Japanese Patent Laid-Open No. 2006-42201